Actress Elizabeth Perkins has blasted the myth the cast of drug comedy Weeds smoke real marijuana on-set - insisting producers supply them with a harmless "stunt herb".

The star plays a suburban housewife with an illegal habit in the hit U.S. TV show.

She reveals the secret behind the substance that doubles for the drug: "It's a combination of herbs that don't get you stoned. If we smoked real weed, we wouldn't get anything done over here."

But Perkins insists the real drug is harmless and should be decriminalised: "I think it is crazy that it is illegal, and I think it is crazy that people are in jail once they get busted with small amounts of marijuana. I just don't see it as that dangerous of a narcotic."